HVAC systems are nearly ubiquitous in modern vehicles such as cars, trucks, and SUVs. Specifically, vehicles use an air conditioner connected to a series of vents in order to blow cool air into the vehicle cabin. With an air conditioner, cooling is generally achieved by circulating refrigerant through a compressor, a condenser, an expansion valve, an evaporator. First, the compressor increases the pressure, and thus the temperature, of the refrigerant and routes it through the condenser as a liquid. Air is blown over the warm compressor to carry heat away from the system, and the liquid refrigerant passes through the expansion valve. As the refrigerant passes through the expansion valve, it suddenly loses pressure causing it to evaporate, thus lowering its temperature. As the cold refrigerant is routed through the evaporator, a fan blows warm air from the area to be cooled, in this case a vehicle's cabin, over the evaporator and the now cooled air is routed back into the area to be cooled. The process repeats as the refrigerant is routed back into the compressor.
Cooling may also be achieved, however, by opening the vehicle's windows. When using open windows to cool a vehicle's cabin, no refrigeration is involved. Instead, cooling is achieved by the airflow caused by the movement of the vehicle.